std::adjacent_find

Defined in header <algorithm>

template< class ForwardIt >
ForwardIt adjacent_find( ForwardIt first, ForwardIt last );

(1)

template< class ForwardIt, class BinaryPredicate>
ForwardIt adjacent_find( ForwardIt first, ForwardIt last, BinaryPredicate p );

(2)

Searches the range [first, last) for two consecutive identical elements. The first version uses operator== to compare the elements, the second version uses the given binary predicate p.

Parameters

first, last	-	the range of elements to examine
p	-	binary predicate which returns `true` if the elements should be treated as equal. The signature of the predicate function should be equivalent to the following: `bool pred(const Type1 &a, const Type2 &b);` The signature does not need to have `const &`, but the function must not modify the objects passed to it. The types `Type1` and `Type2` must be such that an object of type `ForwardIt` can be dereferenced and then implicitly converted to both of them.
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.

Return value

an iterator to the first of the first pair of identical elements, that is, the first iterator it such that *it == *(it+1) for the first version or p(*it, *(it + 1)) != false for the second version.

If no such elements are found, last is returned.

Complexity

Exactly the smaller of std::distance(first, result) + 1 and std::distance(first, last) - 1 applications of the predicate where result is the return value.

Possible implementation

First version

template<class ForwardIt>
ForwardIt adjacent_find(ForwardIt first, ForwardIt last)
{
    if (first == last) {
        return last;
    }
    ForwardIt next = first;
    ++next;
    for (; next != last; ++next, ++first) {
        if (*first == *next) {
            return first;
        }
    }
    return last;
}

Second version

template<class ForwardIt, class BinaryPredicate>
ForwardIt adjacent_find(ForwardIt first, ForwardIt last, 
                        BinaryPredicate p)
{
    if (first == last) {
        return last;
    }
    ForwardIt next = first;
    ++next;
    for (; next != last; ++next, ++first) {
        if (p(*first, *next)) {
            return first;
        }
    }
    return last;
}

Example

#include <algorithm>
#include <iostream>
#include <vector>
  
int main()
{
    std::vector<int> v1{0, 1, 2, 3, 40, 40, 41, 41, 5};
  
    auto i1 = std::adjacent_find(v1.begin(), v1.end());
  
    if (i1 == v1.end()) {
        std::cout << "no matching adjacent elements\n";
    } else {
        std::cout << "the first adjacent pair of equal elements at: "
                  << std::distance(v1.begin(), i1) << '\n';
    }
  
    auto i2 = std::adjacent_find(v1.begin(), v1.end(), std::greater<int>());
    if (i2 == v1.end()) {
        std::cout << "The entire vector is sorted in ascending order\n";
    } else {
        std::cout << "The last element in the non-decreasing subsequence is at: "
                  << std::distance(v1.begin(), i2) << '\n';
    }
}

Output:

The first adjacent pair of equal elements at: 4
The last element in the non-decreasing subsequence is at: 7

unique	removes consecutive duplicate elements in a range (function template)
std::experimental::parallel::ajacent_find (parallelism TS)	parallelized version of `std::ajacent_find` (function template)